Fix Windows I2_S linker errors - #606

Closes #595.

Root cause

ggml.c references quantize_i2_s and dequantize_row_i2_s from the ggml-base target, but their definitions lived in ggml-cpu. Windows links ggml-base independently and therefore reported both symbols as unresolved.

Fix

Update the llama.cpp submodule to isHuangXin/llama.cpp#5, which moves the unchanged platform-neutral implementations into ggml-quants.c (ggml-base) and adds a focused executable that links only ggml-base.

Test verification (RED → GREEN)

• RED, reverted implementation: test-i2-s-linkage fails to link with undefined references to quantize_i2_s and dequantize_row_i2_s.
• GREEN, patched implementation: target builds and the direct executable exits 0.
• ctest --test-dir build/3rdparty/llama.cpp -R "^test-i2-s-linkage$" --output-on-failure: 1/1 passed.
• Full BitNet build: passed.
• Independent code and test reviews: approved.

Dependency: isHuangXin/llama.cpp#5